Memorie
Rivestimenti
Effect of oxide species and bath temperature
on reactions in a galvanizing bath
of Si-containing steel
T. Yasui, M. Nakazawa
Reaction behavior was investigated in a galvanizing bath between Si-containing steel and molten Zn, in
order to understand the influence of Si oxides and solute Si on galvanizing reactions. For the 0.2Si steel, only
Mn2SiO4 formed on the surface of the substrate after reduction annealing. After galvanizing, the Fe amount
in the coatings slightly increased with the rise of the bath temperature. In contrast, the Al amount at the
substrate/coating interface (interfacial Al) decreased. As such behavior is similar to 0.01Si steel, it was
suggested that the influence of Mn2SiO4 on a galvanizing reaction is small. For the 1.2Si steel, on the other hand,
SiO2 formed on the surface of the substrate in addition to Mn2SiO4 after reduction annealing, and the galvanizing
reaction was quite different from other kinds of steel. Although both the Fe in the coatings and interfacial Al were
much lower below 450ºC, the Fe in the coatings increased sharply with a rise in bath temperature to more than
460ºC. As SiO2 was detected on the coating/substrate interface, even after galvanizing at 470ºC for the 1.2Si steel,
it was considered that SiO2 exhibits a barrier effect on the reaction in the bath.
Furthermore, for the 1.2Si steel, the Fe-Zn compounds seemed to form easily compared to other kinds of
steel. It was implied that the balance of stability between the Fe-Zn compounds and Fe-Al compounds was changed
by the solute Si in the substrate in addition to Si oxides.
Keywords: Si-containing steel, high-strength steel, selective oxidation, galvanizing,
Fe-Al compounds, Fe-Zn compounds, SiO2, Mn2SiO4, reaction in the bath
INTRODUCTION
Recently, due to increased demand for automobile safety, the use
of high-strength steel has become necessary in order to achieve
both lightweight design and crash safety. Among the various
kinds of high-strength steel available, multiphase steel, such as
DP steel and TRIP steel, exhibit an excellent strength-elongation
balance. In the case of cold-rolled steel, Si is an ideal element to use when producing such multiphase steel. On the
other hand, galvannealed steel (GA) has been widely used for
automotive bodies from the perspective of corrosion resistance.
However, when Si is added to the substrate of GA, problems,
such as bare spots on the coatings and a galvannealing
delay, are seen. Therefore, Si-containing GA steel is quite difficult to produce.
Recent intensive investigations [1–3] have showed that Si-containing oxides (hereinafter referred to as “Si oxides”), which are
formed during reduction annealing, exhibit poor wettability with
molten Zn, causing bare spots. However, the mechanism for
the delay of galvannealing has not yet been clarified despite
that some theories [4–6] have proposed. Recently, authors [7]
proposed that though Si oxides certainly can delay an alloying
reaction, the galvannealing rate is still low, even for Si-oxide-free
substrate in the case of high-Si steel.
To solve the mechanisms of bare spots and galvannealing
delay, it is crucial to understand the galvanizing reaction in
a bath between substrate with Si oxides/solute Si and molten
T. Yasui, M. Nakazawa
Takeshi Yasui and Makoto Nakazawa
Nippon Steel Corporation, Japan
La Metallurgia Italiana - n. 1/2012
Zn, firstly. In the case of mild steel, the following theories concerning a reaction in a galvanizing bath have been commonly
proposed:
1) Fe dissolves from substrate into molten Zn immediately after
dipping.
2) Dissolved Fe crystallizes into the Fe-Al compounds at the substrate/molten Zn interface at first, and then the compounds
inhibit further Fe dissolution from the substrate.
3) Extra Fe forms the Fe-Zn compounds on the Fe-Al compounds.
In this study, the galvanizing reaction of Si-containing steel,
especially the formation behavior of Fe-Zn/Fe-Al compounds,
were focused on. The purpose of this study is to investigate the
influence of Si oxides and solute Si on galvanizing behavior.
EXPERIMENTAL
Sample preparations
The steel used as substrates in this study consisted of coldrolled steel sheets with the chemical compositions listed in
Table 1. A laboratory hot-dip galvanizing simulator was used to
Steel
C
Si
Mn
0.01Si
0.2Si
1.2Si
0.001
0.1
0.1
0.01
0.2
1.2
0.1
1.5
1.5
TAB. 1
Steel compositions used as substrates (mass%).
Composizioni degli acciai utilizzati come substrato
(massa%).
23
Memorie
FIG. 1 GDS depth profiles of annealed specimens.
FIG. 2
FT-IR RAS spectra
of annealed
specimen.
Spettri FT-IR RAS
dei provini ricotti.
compose the galvanizing tests. First, the substrates were annealed in a reduction atmosphere of N2-5vol.%H2 at 800ºC for 60
s. Then, the substrates were dipped in a Zn-0.13mass%Al0.03mass%Fe bath for 3 s immediately after annealing. Bath
temperatures were controlled to 430, 450, 460, and 470ºC,
respectively. After that, annealed specimens and galvanized specimens were analyzed.
Analysis of substrates and coatings
Glow Discharge Spectroscopy (GDS) and Fourier Transform
Infra-Red Spectroscopy (FT-IR) were used to analyze the surface
oxides of the annealed specimens. Galvanized coatings were
dissolved in a 10vol.% HCl solution containing a 0.2vol.% inhibitor. Then, the Fe amount in the coatings was measured using
Inductively Coupled Plasma Spectroscopy (ICP). On the other
hand, in order to expose the Fe-Al compounds at the coating/substrate interface, galvanized coatings were dissolved
electrolytically at -650 mV versus an Ag/AgCl electrode in
a 13mass%NH4Cl solution. Then, the Al amounts remaining
on the substrate (interfacial Al) were measured using X-ray
Fluorescence Spectroscopy (XRF).
The cross-sectional microstructures of the galvanized specimens were observed by optical microscope (OM) and a Field
Emission Scanning Electron Microscope (FE-SEM).
The surface morphology of the Fe-Zn compounds was observed
using FE-SEM after the dissolution of the η phase in the
0.5vol%HCl solution.
FT-IR was also used to analyze the surface oxides of galvanized
specimens after dissolution of the entire coating.
Profili di profondità GDS dei provini ricotti.
Mn and Si, and O were enriched on the surface. The Si peak
height was higher for 1.2Si steel than 0.2Si steel, although the
Mn peak heights were almost the same. Figure 2 shows the FTIR RAS spectra of the specimens after annealing. Peaks at 1,000
cm-1 were attributed to Mn2SiO4, and peaks at 1,250 cm-1 were attributed to SiO2 [8]. For the 0.01Si steel, no oxides were detected.
For the 0.2Si steel, only Mn2SiO4 was detected, while both
Mn2SiO4 and SiO2 were detected for the 1.2Si steel.
Change of coating chemistry and microstructure
Figure 3 shows the change of the Fe in the coatings as a
function of the bath temperature. It is considered that the Fe
in the coatings refers to the Fe amount dissolved from substrate
into the bath during dipping. For the 0.01Si steel and 0.2Si steel,
the Fe in the coatings slightly increased with the rise of the bath
temperature up to 460ºC. Only for the 0.01Si steel, the Fe in the
coatings sharply grew at 470ºC. For the 1.2Si steel, on the other
hand, the Fe in the coatings was much lower when the bath temperature was below 450ºC, but rose rapidly at more than 460ºC.
FIG. 3
Relationship
between the Fe in
the coatings and
bath temperature.
Rapporto fra Fe nel
rivestimento e
temperatura del
bagno.
FIG. 4
Relationship
between
interfacial Al and
bath temperature.
Rapporto fra Al
interfacciale e
temperatura del
bagno.
RESULTS
Surface analysis of annealed specimens
Figure 1 shows the GDS depth profiles of the substrates
after reduction annealing. For the 0.2Si steel and 1.2Si steel,
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La Metallurgia Italiana - n. 1/2012
Rivestimenti
FIG. 5
Cross-sectional OM
images of galvanized
specimens.
Immagini al microscopio
ottico della sezione
trasversale di provini
galvanizzati.
FIG. 6
Surface morphology
of Fe-Zn compounds.
Morfologia superficiale di
composti Fe-Zn.
Figure 4 shows the change of the interfacial Al as a function of
the bath temperature. It is considered that interfacial Al refers
to the Al amount contained in the Fe-Al compounds that formed
in the bath during dipping. For the 0.01Si steel, interfacial
Al decreased with the increase of the bath temperature, and
then hardly remained at 470ºC. For the 0.2Si steel, like the
0.01Si steel, the interfacial Al declined with the rise of the
bath temperature, but still existed at 470ºC—unlike 0.01Si steel. For the 1.2Si steel, however, the interfacial Al was much
lower when the bath temperature was less than 450ºC, but it
grew slightly at more than 460ºC—opposite to the other steel.
Figure 5 shows the cross-sectional images of the galvanized specimens as observed by OM. For the 0.01Si steel, changes of the
microstructures with the increase of the bath temperature was
slight—up to 460ºC—and columnar compounds at the substrate/coating interface (arrowed) were believed to be Fe-Zn
compounds. (Throughout this paper, these columnar phases
are considered to be Fe-Zn compounds as long as no annotation exists.) At 470ºC, the outburst structure was observed locally. For the 0.2Si steel, there seemed to be no big difference in
the morphology and distribution of Fe-Zn compounds, even
though the bath temperature was raised. For the 1.2Si steel, the
Fe-Zn compounds were few at less than 450ºC, but coarse Fe-Zn
compounds existed at more than 460ºC, in contrast.
La Metallurgia Italiana - n. 1/2012
Influence of Si oxides
Figure 6 shows the surface morphologies of the Fe-Zn compounds that were observed in the coated specimens after the
dissolution of the phase. For the 0.2Si steel, the change of the
morphologies of the Fe-Zn compounds with the variation of the
bath temperature was not clear. For the 1.2Si steel, Fe-Zn compounds existed quite locally up to 450ºC. On the other
hand, the size of the Fe-Zn compounds became coarse at more
than 460ºC.
Figure 7 shows the FT-IR RAS spectra taken from the galvanized
FIG. 7
FT-IR RAS spectra
of galvanized
1.2Si steel after
dissolution of the
coatings.
Spettri FT-IR RAS
dell’acciaio zincato
1.2Si dopo
dissoluzione del
rivestimento
25
Memorie
FIG. 8
Cross-sectional SEM
microstructure of
galvanized 0.2Si steel.
Microstruttura al SEM della
sezione trasversale dell’
acciaio 0.2Si galvanizzato.
FIG. 9
Cross-sectional SEM
microstructure of
galvanized 1.2Si steel.
Microstruttura al SEM della
sezione trasversale dell’
acciaio 1.2Si galvanizzato.
1.2Si steel after the dissolution of the entire coating. For the
1.2Si steel, peaks from Si oxides were detected for the substrate galvanized at any bath temperature. The peak heights
of Mn2SiO4 became lower with the rise of bath temperature;
then there was no peak from Mn2SiO4 at 470ºC. However, the
peak from SiO2 still survived even after galvanizing at a bath
temperature of 470ºC. Although the same analyses were performed for the 0.01Si steel and the 0.2Si steel, no peaks of Si oxides were detected at any bath temperature.
Figure 8 shows the cross-sectional SEM microstructure for
the galvanized 0.2Si steel. At 430ºC, Fe-Al compounds formed
at most of the substrate/coating interface. Si oxides that were
captured in the Fe-Zn compounds were also observed in some
areas. At 450–470ºC, the area where the Fe-Al compounds formed reduced with the increase of bath temperature. In contrast,
the area where the Si oxides were captured in the Fe-Zn compounds increased. At any bath temperature, no evidence of residual Si oxides at the initial substrate/coating interface was
found.
Figure 9 shows the cross-sectional SEM microstructure for the
galvanized 1.2Si steel. Compared to the 0.2Si steel, the microstructure was very distinctive at a bath temperature of 430ºC;
columnar Fe-Al compounds formed from the substrate/coating
interface locally. Except for the area where Fe-Al compounds for-
26
med, Si oxides remained at the initial substrate/coating interface. These results corresponded to the FT-IR spectra (Fig.7).
At 450–460ºC, hollow areas (circled) where Si oxides were supposed to be captured in the coatings were observed. At 470ºC,
such hollow areas on the substrate were spread.
DISCUSSION
In this study, the galvanizing reaction of Si-containing steel
was investigated and the following results were obtained:
1) The amount and morphology of Fe-Al/Fe-Zn compounds that
formed during galvanizing changed with the variation of the
bath temperature; and
2) The situation of the change differed according to the Si content in the substrate and species of Si oxides present.
The reason for such results is discussed below.
Change of stability of Fe-Al/Fe-Zn compounds
From the results of the chemical analyses, it was considered that
the increase of the Fe amount that was dissolved into the bath
was minimal, regardless of the bath temperature for the 0.01Si
steel and 0.2Si steel (Figure 3, except for the plot of the
0.01Si steel at 470ºC). The reason for this was believed that
the diffusion coefficient of the Fe in the molten Zn did not
change substantially with the rise of the bath temperature from
La Metallurgia Italiana - n. 1/2012
Rivestimenti
430—470ºC. In contrast, the amount of Fe-Al compounds that formed at the substrate/coating interface when galvanizing was
supposed to decrease with the increase of the bath temperature (Figure 4). The reason for this was believed to be due to the
fact that Fe-Al compounds would be more stable at lower temperatures for the Zn bath composition used for galvanizing,
while the relative stability of Fe-Zn compounds would become higher with the increase of the bath temperature [9].
In contrast, for the 1.2Si steel, even though the galvanizing
reaction hardly occurred at a bath temperature of 430ºC and
450ºC, the Fe amount dissolved into the bath increased rapidly
at more than 460ºC, and then the amounts of the Fe-Zn increased. From these results, it was supposed that the influence of Si
oxides and solute Si on the galvanizing reaction was dominant
for the 1.2Si steel.
Effect of species of Si oxides
For the 0.2Si steel, although Mn2SiO4 formed on the surface of the substrate after reduction annealing, the galvanizing reaction was similar to that of the 0.01Si steel. In
other words, the influence of Mn2SiO4 on the galvanizing reaction was small. From the SEM microstructure (Figure 8), it
was supposed that since Mn2SiO4 was captured into the
compounds during dipping, the influence on the galvanizing
reaction would become much less.
In contrast, for the 1.2Si steel, as SiO2 survived at the surface of
the substrate even after galvanizing (Figure 7), SiO2 was believed to have a large barrier effect on the reaction between the
substrate and molten Zn. Moreover, it was expected that the barrier effect would become much larger at a lower bath temperature, as the Fe in the coatings was low, below 450ºC.
Galvanizing reaction for high-Si-containing steel
For the 1.2Si steel, even though the Fe in the coatings was higher
than the 0.2Si steel, at more than 460ºC, the interfacial Al was
lower than the 0.2Si steel. This suggested that, for the 1.2Si steel,
the stability of the Fe-Zn compounds became relatively higher
when compared to the Fe-Al compounds through the following
two possible hypotheses: (a) Fe solubility in molten Zn declines rapidly through the dissolution of Si into molten Zn
[10], or (b) the diffusion coefficient in the Fe-Al compounds
falls by containing Si [11].
CONCLUSION
In this study, the influence of Si oxides and solute Si on the galvanizing behavior of Si-containing steel was investigated. As a
result, the following conclusions can be drawn:
• For the 0.01Si steel, although the Fe in the coatings slightly increased with the rise of the bath temperature, the interfacial
Al decreased gradually. Then at the bath temperature of
470ºC, the interfacial Al were hardly remained and the outburst structure was observed.
• For the 0.2Si steel, although Mn2SiO4 formed on the surface of the substrate after reduction annealing, the galvanizing reaction was similar to that for the 0.01Si steel.
• For the 1.2Si steel, both SiO2 and Mn2SiO4 formed on the
surface of the substrate after reduction annealing, upon
which the Fe in the coatings and interfacial Al were
shown to be much smaller at a lower bath temperature. At a
higher bath temperature, the Fe in the coatings sharply increased while the interfacial Al rose slightly.
• For the 1.2Si steel, the morphology of the Fe-Zn compounds and Fe-Al compounds was distinctive compared to
other kinds of steel. It was suggested that the species of
Si oxides present or that the solute Si in substrate contributed to such behavior.
REFERENCES
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’04, (2004), p 529.
3) P. Drillet, Z. Zermout, D. Bouleau, J. Mataigne and S. Claessens: Galvatech ’04, (2004), p 1123.
4) A. Nishimoto, J. Inagaki and K. Nakaoka: Tetsu-to-hagane, 68(1982),
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Abstract
Effetto delle specie di ossido e della temperatura del bagno
sulle reazioni in un bagno di zincatura di acciaio contenente Si
Parole chiave: rivestimenti, acciaio
In un bagno di zincatura è stata studiata la reattività tra l’acciaio contenente Si e lo zinco fuso, al fine di comprendere l'influenza
degli ossidi di Si e del Si in soluzione sulle reazioni della zincatura. Per l'acciaio 0.2Si, sulla superficie del substrato si forma solo
Mn2SiO4 dopo ricottura riducente. Dopo la zincatura, la quantità di Fe nei rivestimenti aumenta leggermente con l’innalzamento
della temperatura del bagno. Al contrario, la quantità di Al all’ interfaccia substrato / rivestimento (Al interfacciale) diminuisce.
Poiché tale comportamento è simile a quella dell'acciaio 0.01Si, è stato suggerito che l'influenza di Mn2SiO4 su una reazione di
zincatura sia ridotta. Per l'acciaio 1.2Si, invece, sulla superficie del substrato si è formato SiO2 in aggiunta a Mn2SiO4 dopo ricottura
riducente, e la reazione di zincatura è stata molto diversa rispetto agli altri tipi di acciaio. Sebbene sia Fe nei rivestimenti sia l’
Al interfacciale siano molto più bassi al di sotto dei 450 ° C, l’ Fe nei rivestimenti risulta aumentato nettamente con un aumento
della temperatura del bagno oltre i 460 ° C. Per l'acciaio 1.2Si l’ SiO2 è stato rilevato sull’ interfaccia rivestimento / substrato, anche
dopo la zincatura a 470 ° C, quindi si è ipotizzato che l’ SiO2 abbia un effetto barriera sulla reazione nel bagno.
Inoltre, per l'acciaio 1.2Si, i composti Fe-Zn sembrano essersi formati più facilmente rispetto ad altri tipi di acciaio. Ciò suggerisce che il bilancio di stabilità tra composti Fe-Zn e composti Fe-Al è stato cambiato dall’ Si in soluzione nel substrato oltre agli
ossidi di silicio.
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